freezing if i leave pc alone

its like the 3 time i had to install xp pro on my hd and every time i did, ive did the long formating way and after every time i installed a clean xp its been getting slower and slower so now it just freezes up if im not doing nothing on the pc and some time it freezes up when i am. it would slowly start to freeze up, like first the program would stop responding but can still click on stuff but just doesent respond then cant click nothing then the mouse stops moving. this is a fix that ive tryed to do by scaning the hole pc for vireses and what not, i use panda, malwarebytes, spybot seach and destroy and its all good. dont know if its the ram becouse i just replaced my old ones.on the last xp i had before i reinstalled i had a nasty vires that i got rid of but came back then got rid of again then reinstalled xp. on my new xp thers no signs of it thow.

i was thingking it might be the hd couse for some reason its reading my hd like if it was a removeable drive. bottom right icons tells me i can safely remove my hard drive and it list my hd model number. its a western digital SATA II CAVIAR GREEN dont know if that helps.